Physical inactivity is associated with increased cardiovascular (CV) risk and mortality. Adequate sleep is essential for health, but paradoxically, sleep also represents our longest period of inactivity and could therefore compromise circulatory function. Indeed, adverse CV events, including sudden cardiac death, and myocardial infarction all have their highest incidence in the morning, soon after awakening i.e., immediately following this longest period of inactivity that accompanies sleep. In healthy humans, my mentor's research team discovered that our internal body clock causes many (CV) disease risk markers to peak at this time in the morning. These markers include sympatho-vagal responses to stress, markers of thrombosis or clot formation, and the stress hormone cortisol. Additionally, diurnal studies indicate that vascular endothelial function (VEF), which is a strong predictor of CV disease, is also impaired in the early morning. This increased morning risk could be due to individual effects on vascular function of prior sleep or inactivity (so-called `behavioral effects), an internal circadian rhythm, or an interaction between behavioral and circadian effects. To determine whether the inactivity or sleep does pose a potential risk factor, in particular in regards to vascular function, the applicant plans to assess the effects of sleep and inactivity on arterial VEF and, the effect of the endogenous circadian system on vascular function. Our specific aims are to: 1) determine if endogenous circadian rhythms in VEF during rested wakefulness show a trough around 9 AM; and 2) test the separate effects of physical inactivity and sleep on VEF. Our exploratory aims are to: 1) determine the effect of a standardized day on VEF at different circadian phases; and 2) determine the effect of increased physical activity on VEF immediately after awakening. Although the applicant brings expertise in VEF to the laboratory, other dependent CV risk markers will also be measured as part of the associated R01-funded project, including measures of hemodynamics and autonomic nervous system activity. To document circadian rhythms, subjects will spend 5 days in a laboratory in a constant environment and will live on recurring 5h 20min days during which behavior is tightly controlled. To document effects of sleep and inactivity, participants will spend two nights in the laboratory in two experimental conditions in a randomized order: 1) sleep combined with inactivity and 2) wakefulness combined with inactivity. By assessing novel mechanisms by which circadian rhythms and inactivity can affect CV risk, this study will set the foundation for future preventive interventions that could include passive motion during sleep, extending the habitual timing of sleep to avoid risky activities during the vulnerable time window, and to study populations at risk of CV disease, such as those with prediabetes, prehypertension, elderly, obese and those who are physically inactive. This project will set the basis for research that will in the long term, enhance the health of all individuals so that they can live longer and more fulfilling lives.